Used aluminum beverage cans and other products made from aluminum are often recycled to recover the aluminum they contain for use in other products. Typically, aluminum scrap is recycled in reverberatory furnaces. Flux is used in these furnaces to promote the melting of the aluminum. This flux, together with oxides, dirt, and other materials, forms a viscous mass of dross, which floats on top of the molten aluminum. This dross impedes the assimilation of additional metal to the molten aluminum, and therefore must be skimmed off of the liquid aluminum before additional solid material may be melted in the furnace. The dross solidifies after cooling into a mass that typically contains about 65% aluminum metal.
Methods to recover aluminum from furnace dross include grinding and screening the metallic dross. Grinding achieves some separation of the metal from the portions having a high salt and oxide content because the waste products are more friable than the portions having a relatively high metal content. Therefore, after grinding, the portions of the dross constituting waste are reduced to fine particles, while those portions having a higher aluminum content tend to resist reduction by the mill. Accordingly, a screening operation can achieve some separation between metal enriched and waste products. However, previous methods for recovering aluminum values from dross that rely on grinding processes either do not produce a high enough grade product because efficient separation of aluminum from waste is not effected or have poor recoveries if sufficient size reduction is conducted because aluminum fines are produced which are lost with waste product.
Another method for directly recovering metal from metal-containing dross, used alone or in combination with grinding, is the melting of metal-containing dross in a rotary furnace. According to such a method, the dross is fed into a rotary furnace, together with a large amount of salt flux. The flux is necessary to enable the release of the aluminum metal from the other constituents. This salt flux reports to salt cake upon cooling, which must be landfilled. Therefore, such methods have associated drawbacks, such as environmental concerns.
An additional method for recovering metal from dross, disclosed in U.S. Pat. No. 5,192,359, involves the separation and classification of dross particles based on the electrical conductivity of each particle. According to this method, size separation is conducted and large particles are processed in a furnace and small particles are subjected to a linear electromagnetic force field provided by an inclined linear induction motor. Particles that are dropped generally along the length of the inclined linear motor are levitated if they are conductive. A separator then allows the conductive, metal-containing particles to be collected separately from the waste particles.
Accordingly, there is a need for an economical process that is capable of recovering high grade metal from metal-containing dross, and which is capable of processing large amounts of dross. Furthermore, there is a need for a process that reduces the amount of waste material remaining after the metal has been removed from the metal-containing dross.